In contrast to continuous brake systems which are used, for example, in trucks, semi-continuous brake systems are used in agricultural tractors. Such semi-continuous brake systems include two energy sources that are operated independently of one another, wherein one energy source is assigned to a tractor brake system and the other energy source is assigned to a trailer brake system. It is furthermore known to provide traction vehicles such as, for example, agricultural tractors, trucks, semi-trailer trucks, construction vehicles or other vehicles designed for towing or hauling, with coupling devices for trailer brake systems, to which a corresponding trailer coupler of the trailer brake system can be coupled when the trailer is connected to the tractor. A pressure-conducting connection between the tractor and the trailer can be produced by connecting the coupling devices of the tractor and the trailer, wherein this connection makes it possible to operate, for example, a trailer brake system in a pressure-controlled or pneumatic fashion. The required compressed air is generated with a compressor that is arranged on the tractor and driven by a drive unit, for example, via a belt drive that is connected to the engine of the tractor.
It is common practice for the tractor to continuously generate the system pressure required for a trailer brake system regardless of whether a trailer is coupled to the tractor. In order to conserve energy and fuel, however, the compressor is operated only when the system pressure for the trailer brake system drops below a predetermined nominal value. In this respect, corresponding sensors and pressure control devices may be provided, wherein the compressor controlled by means of an electronic control unit can be switched on and off in dependence on said sensors and pressure control devices. For example, the compressor is switched off when a predetermined system pressure is reached. The system pressure for the trailer brake system also drops if no trailer is coupled to the tractor, for example, due to the actuation of the tractor brake system or operational leaks in the tractor. In this case, the system pressure is once again raised to the level of the predetermined nominal value by switching on the compressor in a controlled fashion. Consequently, the compressor is also repeatedly switched on during the operation of the tractor when no trailer is coupled to the tractor such that unnecessary losses of energy occur and the fuel efficiency deteriorates.